We have analyzed the trajectory of 800 microshperes as they flowed through 46-97u diameter arterial vessels in a single membrane hamster cheek pouch preparation. The null hypotheses were that 15u carbonized microspheres either follow a homogeneous density distribution or centrally peaked density distribution. The experimental results showed, instead, a significant peripheral peak in the density distribution, which significantly (P is less than 0.001) differed from the null hypotheses. This finding has not been previously suspected from either experimental engineering models or animals studies. Various linear regression analyses with respect to arterial endothelial radius, microsphere velocity or analysis of geometric factors did not elucidate the cause of this greater than expected number of microspheres towards the periphery of the accessible vessel lumen.